Go Nil

What is nil in Go

nil in Go has several meanings:
– It represents “null” in Go. This means two things: 1. It does not have type. 2. Its value is “null”.
– It is a predeclared identifier in Go, which means you can use it without declaring it.
– It represents zero values (and default values) of some types in Go, including:

Take var sb = (map[string]bool)(nil) == (map[string]bool)(nil) as an example, the reason why two nil values of a same type (map[string]bool) are not comparable is because Go does not support comparison in slice, map and function types. You can see that we are comparing two values of a non-comparable type in this case. That is why it fails.

Take var sb = (map[string]bool)(nil) == nil as an example, (map[string]bool)(nil) declares a map[string]bool temporary variable which value is nil and (map[string]bool)(nil) == nil detects whether the variable’s value is nil and then assigns the results to sb. You can see that we are comparing the value of a non-comparable type with its zero value (nil) in this case. That’s why it works.

Two nil Values of The Same Type Can Be Comparable Only When This Type Supports Comparision

Be Careful in nil Comparision When Interface Values Are Involved

Explanation:
– An interface value consists of a dynamic type and a dynamic value.interface{}(nil) declares an interface value with {type: nil, value: nil}.
– The non-interface value is converted to the type of the interface value before making the comparison with an interface value. In this example, (*int)(nil) is converted to an interface value with {type: *int, value: nil}.
– Two nil interface values are equivalent only when they carry the same type. In this case, the converted interface value {type: *int, value: nil} has a concrete dynamic type but the other interface value has not. That is why the comparison result is false.

Explanation:
– An interface value equals to nil only when its type and value are both nil. In example, w is an io.Writer interface value with {type: *bytes.Buffer, value: nil} after the w = b assignment. Therefore, w == nil is false as it carries *bytes.Buffer other than nil as its concrete dynamic type.

Summary

nil is and a pre-declared identifier which can be used to represent the zero values of some types in Go.

Be careful when using nil in comparison, especially when interface values are involved. You need to understand what you are comparing: types, or values, or both.

(a thing)(nil) may not equal to nil, depends on what that thing is (a pointer or an interface). It means Go is a strong-type language and this also applies to nil even though nil itself does not have default type (sarcasm).